Ink jet printing is a conventional process used to print an image, typically in the form of digital information from a computer or the like, onto a substrate, such as paper. Specifically, this process provides a non-impact means for generating images by computer control over the direction of small droplets or particles in rapid succession onto the substrate. A primary ink jet printing system is a continuous jet system, where ink is forced under pressure from a single or multiple nozzles. The ink tends to form a droplet stream of a size and frequency determined mainly by the surface tension of the ink liquid, the application pressure and the nozzle(s) size(s).
FIG. 1 shows an example of a contemporary ink jet system 10. Ink from an ink source 11 is moved by a pump 12 through a filter 13 to a piezoelectric crystal driver 14, along a common line 15. The line 15 terminates in a nozzle 16, such that the ink breaks into droplets upon being forced, by the piezoelectric crystal driver 14, through the nozzle 16. The droplets form a dot stream 17 that moves through a charged unit 18 and then through high voltage deflection plates 19, that charge and deflect the ink stream 17 into respective charged 17a dots and deflected dots 17b, in accordance with a digital signal corresponding to the image to be printed. Dots in the charged dot stream 17a, are charged to different amounts, corresponding to the desired deflection, that once past the mist guard 20, contact the substrate 21 (e.g., paper) in a manner so as to form characters thereon. The substrate 21 is in turn, mounted on a drum 22 or the like, that rotates in the direction of the arrow 23. Dots of the deflected stream 17b exit the system in the direction of arrow 24, through a gutter or the like, and the ink, from which these dots are formed, may be returned to the ink source 11 if desired.
This continuous ink jet printing apparatus, as well as other conventional ink jet systems, such as drop on demand systems, employ water based inks, such as Ink No. 1007 used in model No. 5240 Ink jet printer, from Scitex Digital Printing, Inc., Dayton, Ohio, USA, and inks for the Smartjet 4012 continuous ink jet printer, from Iris Graphics, Inc., Bedford, Mass., for continuous ink-jet printers, and inks, such as those used in cartridge model numbers HP51626A, HP51625A, HP51629A and HP51649A for the HP DESK JET.RTM. and DESKWRITER.RTM. printer series, from Hewlett-Packard Company, Palo Alto, Calif., and those inks in cartridge model numbers S020034 and S020036 for the Epson.RTM. STYLUS.TM. printer series Color Ink Printer, from Seiko Epson Corporation, Japan, for drop on demand ink jet systems. Specific viscosities usually depend on the particular ink jet technology employed. For example, viscosities ranging from 1 to 10 centipoise are common for continuous ink jet inks and viscosities up to 30 centipose are common for drop on demand ink jet inks.
In order to achieve these low viscosities, the inks often contain a water soluble dye and have low viscosity additives that function to enhance performance and stability. The water soluble ink with the dye is such that the dye is required to be soluble in the ink, such that the ink dries rapidly once it is on the substrate (paper). If drying is too slow, smearing typically occurs by a subsequently printed sheet landing on the previously printed but still drying sheet, or if the wet or dry finger of an operator contacts the printed portions. Additionally, even in documents, days, weeks months, or even years old, a wet finger or a dry finger with sufficient pressure may be sufficient to smear the print on the document. Thus, as a result of the ink failing to properly dry, this gives rise to anesthetic documents, that in most cases must be reprinted, if possible, wasting both labor and materials.
Several attempts have been made at solving these problems. One approach employs a long drying oven, while another approach uses a special paper pretreated with a composition for fixing the dye. These attempts have substantial drawbacks.
In particular, the drying oven is expensive as it involves costly equipment and occupies a large amount of floor space. The special paper is also expensive, for it is a treated paper.
Ink jet printing is a relatively new process when it is compared with one of the most popular printing processes which is offset lithography. A fundamental difference between the two processes is that offset lithography requires a pre-imaged plate as a master, from which multiple copies are made. Therefore, ink jet printing can easily produce variable information from print to print, whereas this is not possible with pre-imaged offset plates. However, the nature of offset printing permits the use of very high viscosity inks and provides means of avoiding set-off for both line-work and continuous tone color on a variety of papers, including hard surface high quality coated paper for best quality printing. Because such offset inks are oil based and often contain air oxidisable oils, they are resistant to rubbing, especially after a few hours. However, these inks have not been adapted for use in ink jet printing processes.